Because the lungs are a major interface between the body and its environment, airborne particulates can present health risks to workers in dusty environments. Well known examples of occupationally related pulmonary diseases include coal worker's pneumoconiosis (CWP) induced by exposure to coal dust, silicosis induced by exposure to particles of crystalline silicon dioxide (usually encountered in nature as quartz), and asbestosis caused by exposure to environmental asbestos particles. To reduce the incidence of severe long-term health effects of such exposures, occupational regulations mandate monitoring exposure to particulates in certain work environments.
For example, the Federal Coal Mine Health and Safety Act of 1969 and the Federal Mine Safety and Health Act of 1977 mandated measurement of personal exposure to coal mine dust in the United States. A number of air sampling devices have evolved in response to this type of requirement. A common approach is to use small cyclone air samplers (such as those available through Mine Safety Appliances Co., Pittsburgh, Pa., or SKC, Inc., Eighty Four, Pa.) that can be placed in the work environment or attached to a worker. The cyclone separates smaller respirable particles from larger nonrespirable particles, and collects the respirable particles on a filter for subsequent gravimetric analysis.
The collection of dust particles for subsequent gravimetric analysis can provide an indication of a total particulate exposure over the collection period, but this technique is unable to provide real-time measurements of dust exposure as it occurs. Over the last few decades, several methods have been developed to provide a continuous direct reading of airborne particulate mass concentration. These techniques have included light scattering nephelometry, absorption photometry, beta radiation attenuation (in which beta radiation is attenuated by interaction with electrons in atoms of particulates), measurement of a pressure differential across a sampling tube into which dust particles are drawn and collected (as in U.S. Pat. No. 6,401,520), and resonance frequency decrement.
Resonance frequency decrement devices provide a mass in combination with an energy storage device (such as a spring, or electromagnetic equivalent) that oscillates the mass harmonically at its natural resonance frequency, which is typically proportional to the square root of the ratio of the system stiffness constant and its mass. As this mass increases (during particle collection), the system resonant frequency diminishes. This frequency decrement is a measure of collected particle mass. Examples of this technique (which are further described in U.S. Pat. No. 5,349,844) include an oscillating wire or ribbon used as a particle impaction surface, a quartz crystal piezo-balance, a tapered element oscillating microbalance (TEOM®), and an oscillating filter tape monitor. An advantage of resonant mass monitoring devices is that they provide a continuous direct mass sensing technique with the closest approximation to a reference gravimetric method.
The TEOM® technique was pioneered by Rupprecht & Patashnick Co. (now part of Thermo Electron Corp., pending part of Thermo Fisher Scientific, Inc.) as illustrated in U.S. Pat. No. 3,926,271. In that early approach, the microbalance had a tapered tubular element clamped at one end while the other end was free to vibrate a mass measurement platform. The tapered tubular element was set into oscillation and a feedback system maintained the oscillation. A change of mass on the oscillating platform was determined by measuring the resonant frequency of the tapered tubular element, which changed in mathematical relationship to the mass loading of the tapered tubular element.
A variety of resonance frequency microbalances have been developed in recent years. For example, U.S. Pat. No. 5,349,844 disclosed a resonance frequency microbalance in which a particulate collection filter membrane was oscillated perpendicular to the plane of the filter. In other work, the Rupprecht & Patashnick (R&P) TEOM® has also been developed, in one case into a device that can be used for EPA-required monitoring of particulate concentration in outdoor environments (Patashnick and Rupprecht, J. Air Waste Management Assoc. 41:1079, 1991). TEOMs may additionally be adapted for use in a variety of settings, including workplaces. A TEOM® monitor draws in an air sample that is typically heated to about 45° C. to reduce relative humidity, and particles are subsequently collected on a Teflon® and fiberglass filter that is held by and oscillates with a mounting platform on the tip of a glass or metal tube. The specialized Teflon® and fiberglass filter of a TEOM® monitor has been designed to provide a very hydrophobic matrix that minimizes the collection of airborne moisture that may otherwise collect on the filter and provide an inaccurately high particulate mass determination.
A recent embodiment of the R&P TEOM® device is the battery-operated Series 3600 Personal Dust Monitor (PDM) which is contained in a compact housing that can be attached to a worker's belt. The PDM separates smaller particles of the respirable size range from larger particles that would less readily reach the lungs when inhaled. The air carrying respirable particles is then flowed through a hydrophobic filter on the mounting platform where the particulate matter is collected, and its mass is determined by a decrement of resonance oscillation of the TEOM.
Although such microbalances are able to accurately determine a mass of collected particulates, they are unable to chemically identify the particles that are collected and measured. For example, it is sometimes helpful to determine the mass concentration of quartz particles in coal mine dust particles of mixed composition because of the increased risk of disease associated with quartz dust exposure. At the present time, the chemical identification of particulates is accomplished by their collection through a separate device (such as a cyclone) on a filter that is then ashed and subjected to spectroscopic analysis. An example of a technique for analysis of quartz in coal mine dust is found in the NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, Method 7603. Another quartz analytical method for the infrared determination of quartz in respirable coal mine dust is provided in Method P-7 from the Mine Safety and Health Administration (MSHA).